1. Field of the Invention
The present invention relates to boundary acoustic wave devices which are, for example, used as band-pass filters. More particularly, the present invention relates to a boundary acoustic wave device which includes an IDT electrode disposed between a first medium and a second medium and which uses boundary acoustic waves propagating at the boundary between the first medium and the second medium.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as resonators and band-pass filters. On the other hand, recently, instead of surface acoustic wave devices, boundary acoustic wave devices have been receiving attention because the package size can be reduced.
For example, PCT International Application Publication No. 2004/070946 discloses a boundary acoustic wave device having a structure shown in the schematic cross-sectional view of FIG. 14. A boundary acoustic wave device 101 includes a first medium 102 and a second medium 103 that are stacked together. A LiNbO3 substrate is used as the first medium 102, and SiO2 is used as the second medium 103. An IDT electrode 104 made of Au is disposed at the boundary between the first medium 102 and the second medium 103.
Since the IDT electrode 104 is made of a metal having a high density and a low acoustic velocity, vibrational energy is concentrated in a portion in which the IDT electrode 104 is disposed, that is, in the boundary between the first medium 102 and the second medium 103, and boundary acoustic waves are excited.
As described in PCT International Application Publication No. 2004/070946, in the structure in which the IDT electrode 104 made of Au is disposed at the boundary between the first medium 102 made of LiNbO3 and the second medium 103 made of SiO2, for example, assuming that the wavelength of boundary acoustic waves is λ, the thickness of the IDT electrode 104 is 0.05λ, and the duty of the IDT electrode 104 is 0.5, the reflection coefficient |κ12|/k0 of the electrode finger is about 0.15, which is relatively high. Note that the reflection coefficient |κ12|/k0 corresponds to the inter-mode coupling coefficient, which is the index of the amount of reflection of the electrode fingers, κ12 represents the inter-mode coupling coefficient based on the mode coupling theory, and k0 represents a wave number 2π/λ of boundary acoustic waves propagating through the IDT electrode. In the structure in which an IDT electrode made of Al is disposed on a LiTaO3 substrate in a conventional surface acoustic wave filter provided in the RF stage of a mobile phone, the reflection coefficient |κ12|/k0 of leaky surface acoustic wave LSAW is only about 0.03 to 0.04.
When the reflection coefficient is relatively high, the stop band in a reflector can be increased. Consequently, with a resonator-type filter in which reflectors are disposed on both sides of the region in which an IDT electrode in the elastic wave propagation direction is provided, the bandwidth can be easily increased. Furthermore, the number of electrode fingers in the reflectors can be decreased, and thus, the size can be reduced.
However, when a resonator-type filter is configured, a pass band is provided in the vicinity of the end of the stop band. Therefore, as the width of the stop band increases, the frequency variation increases due to variations in the line width and thickness of electrode fingers, which causes a problem.
Here, the end of the stop band of an IDT electrode refers to an upper end or a lower end of the stop band when positive and negative terminals of the IDT electrode are short-circuited to provide a grating reflector. The end of the stop band corresponds to the lower end when κ12 is positive and corresponds to the upper end when κ12 is negative.
In the boundary acoustic wave device described in PCT International Application Publication No. 2004/070946, frequency adjustment is performed by adjusting the thickness of the IDT electrode. Consequently, when the thickness of the IDT electrode varies in the manufacturing process, a variation in characteristics due to the thickness variation in the manufacturing process can be minimized by the frequency adjustment. However, in the manufacturing process, the line width of the electrode fingers of the IDT electrode also tends to vary, and it is difficult to control the variation in frequency characteristics due to such a variation in the line width.
Furthermore, for example, when a longitudinally coupled resonator-type filter is configured, the radiation conductance property at a positive κ12 has a peak on the lower side of the pass band of the filter. Consequently, as shown in FIG. 15, the attenuation decreases on the lower side of the pass band, and a large spurious response indicated by the arrow A occurs, which causes a problem.